Mass flow sensors measure and meter fluids, such as, for example, liquids, gases, or slurries. Thermal mass flow sensors are one type of mass flow sensor that determine mass flow via a measurement of heat transfer or energy consumption during a transfer of heat. By way of example, one particular type of thermal mass flow sensor determines flow rate by heating or cooling a fluid as it flows through a conduit and measuring a temperature change in the fluid. For example, a measurement of the fluid temperature upstream of a heating or cooling element can be compared to a measurement of fluid temperature at a location that is downstream of the heating or cooling element. This thermal loss characteristic will vary with mass flow rate. Therefore, the upstream and downstream temperature measurements can be compared in order to determine the amount of heat being carried away from a heating or cooling source. By way of example, the thermal mass flow sensor may output the difference between the upstream and downstream measurements as a thermal sensor signal to one or more electronics, which may, using well known techniques, use the thermal sensor signal to determine the mass flow rate of the fluid.
Since thermal mass flow sensors heat or cool mass for purposes of determining mass flow rate, thermal mass flow sensors while accurate and substantially stable, are inherently slow to respond to a change in flow. For example, the time constant of most thermal mass flow sensor ranges from about two (2) to about four (4) seconds. Therefore, during and shortly after fluctuations in the flow rate of the fluid, it is difficult to calculate an accurate mass flow rate from the thermal sensor signal until the flow rate stabilizes. Although attempts have been made to mathematically accelerate the response time of thermal mass flow sensors, these attempts have largely tended to increase the noise in the thermal sensor signal.
In addition to mathematical acceleration attempts, attempts have been made to mathematically correct mass flow measurements derived from the thermal sensor signal. Using well known techniques, the one or more electronics may then correct the mass flow rate determined from the thermal sensor signal.
However, no existing mass flow measurement device derives its mass flow measurement from more than one type of sensor. In particular, no existing mass flow measurement device derives its mass flow measurement via the combination of a thermal based flow measurement and a pressure based flow measurement. The present invention is directed to a mass flow measurement device that provides accurate measurements during substantially stable conditions and fluctuating fluid flow conditions via the combination of a thermal based flow measurement and a pressure based flow measurement.